Hip and Knee joint replacements are a common and largely successful procedure that utilise implants to restore mobility and relieve pain for patients suffering from e.g. osteoarthritis. However, ions released from both the bearing surfaces and non-articulating interfaces, as in modular components, can cause hypersensitivity and local tissue necrosis, while particles originating from a polymer component have been associated with aseptic loosening and osteolysis. Implant coatings have the potential to improve surface properties compared to both bulk metal and ceramic alternatives. A ceramic coating has the potential to increase scratch resistance, provide greater wettability and reduce wear rates of the counter surface compared to the metallic substrate, whilst maintaining an overall toughness of the bulk implant material and hence lower the risk of catastrophic failure of the device compared to a bulk ceramic material. Coatings can also act as barriers to inhibit ion release from the underlying material caused by corrosion. This review aims to provide a comprehensive overview of coatings that are (i) in current clinical use and (ii) under investigation for future use in joint replacements. While the majority of coatings belong predominantly in the latter group, a few coated implants have been successfully exploited and are available for clinical use in specific applications. Commercially available products include titanium nitride (TiN), titanium niobium nitride (TiNbN), oxidized zirconium (OxZr) and zirconium nitride (ZrN) based coatings, whereas current research is focused not only on these, but also on diamond-like-carbon (DLC), silicon nitride (SiN), chromium nitride (CrN) and tantalum-based coatings (TaN and TaO). The coating materials referred to above that are still at the research stage have been shown to be non-cytotoxic and to reduce wear in a laboratory setting. However, the adhesion of the coatings to the implant remains a main area of concern, as poor adhesion can cause delamination and excessive wear. In clinical applications zirconium implant surfaces treated to achieve a zirconium oxide film and TiNbN coated implants have however been proven comparable to traditional cobalt chromium implants with regards to revision numbers. In addition, the chromium ion levels measured in the plasma of patients were lower and allergy symptoms were relieved. Therefore, coated implants could be considered an alternative, in particular, for patients with metal hypersensitivity. There have also been unsuccessful introductions to the market, such as DLC coated implants, and therefore this review also attempts to summarize the lessons learnt.

Silicon nitride (SiNx) coatings are promising for joint replacement applications due to their high wear resistance and biocompatibility. For such coatings, a higher nitrogen content, obtained through an increased nitrogen gas supply, has been found to be beneficial in terms of a decreased dissolution rate of the coatings. The substrate temperature has also been found to affect the composition as well as the microstructure of similar coatings. The aim of this study was to investigate the effect of the substrate temperature and nitrogen flow on the coating composition, microstructure and mechanical properties. SiNx coatings were deposited onto CoCrMo discs using reactive high power impulse magnetron sputtering. During deposition, the substrate temperatures were set to 200 degrees C, 350 degrees C or 430 degrees C, with nitrogen-to-argon flow ratios of 0.06, 0.17 or 0.30. Scanning and transmission electron spectroscopy revealed that the coatings were homogenous and amorphous. The coatings displayed a nitrogen content of 23-48 at.% (X-ray photoelectron spectroscopy). The surface roughness was similar to uncoated CoCrMo (p = 0.25) (vertical scanning interferometry). The hardness and Young's modulus, as determined from nanoindentation, scaled with the nitrogen content of the coatings, with the hardness ranging from 12 +/- 1 GPa to 26 +/- 2 GPa and the Young's moduli ranging from 173 +/- 8 GPa to 293 +/- 18 GPa, when the nitrogen content increased from 23% to 48%. The low surface roughness and high nano-hardness are promising for applications exposed to wear, such as joint implants.

Ceramic coatings may prolong the lifetime of joint implants. Certain ions and wear debris may however lead to negative biological effects. SiN-based materials may substantially reduce these effects, but still need optimization for the application. In this study, a combinatorial deposition method enabled an efficient evaluation of a range of Si-Fe-C-N coating compositions on the same sample. The results revealed compositional gradients of Si (26.0-33.9 at.%), Fe (9.6-20.9 at.%), C (8.2-13.9 at.%) and N (39.7-47.2 at.%), and low oxygen contaminations (0.3-0.6 at.%). The mechanical properties varied with a hardness (H) ranging between 13.7-17.3 GPa and an indentation modulus (M) between 190-212 GPa. Both H and M correlated with the Si (H and M increased as Si increased) and Fe (H and M decreased as Fe increased) content. A slightly columnar morphology was observed in cross-sections, as well as a surface roughness in the nm range. A cell study revealed adhering pre-osteogenic MC3T3 cells, with a morphology similar to that of cells seeded on a tissue culture plastic control. The investigated coatings could be considered for further investigation due to the ability to tune their mechanical properties while maintaining a smooth surface, together with their promising in vitro cell response.

Silicon nitride-based coatings have been suggested as an alternative for metallic implants in tribological applications as a means to reduce the release of metal ions and debris. However, as silicon nitride is known to slowly dissolve in the presence of water there is a need to quantify and possibly reduce the dissolution rate of the coatings. A previous study found that alloying the silicon nitride coating with Fe and C in order to tune the hardness and elastic modulus did not have a negative effect on surface roughness, the amorphous microstructure nor biocompatibility. However, the ability of the coating to reduce ion release from the substrate as well as the dissolution properties of the coating itself remain to be investigated. Therefore, the aim of this study was to investigate the metal ion release and the dissolution rate of coatings containing Fe and C. 

A combinatorial approach was used for efficient screening of different compositions. Compositional gradients of Si ranging from 36.4 at.% to 47.3 at.%, Fe from 1.4 at.% to 9.3 at.% and C from 4.5 at.% to 13.9 at.% were obtained. Differences in composition did not affect the surface roughness nor the release of Si, Fe or Co ions. Nonetheless, the addition of Fe and C reduced the ion release compared to a silicon nitride reference coating without alloying elements. The introduction of Fe and C might increase the fraction of Si-C or Si-Fe bonds, and thereby reduce the dissolution rate. Preliminary biocompatibility tests showed a fibroblast viability that was comparable to or higher than the uncoated Si substrate.

The possibility to tune the dissolution rate is promising and could benefit coating performance in the intended application, as confirmed by the cytotoxicity tests. 

Ceramic coatings have been widely investigated as a means to reduce wear and metallic ion release from joint implants. Silicon nitride-based coatings have been a topic of interest specifically due to their solubility in aqueous solutions. This could imply a reduced adverse immune response since the generated debris would dissolve. However, there are concerns regarding the dissolution rate and adhesion of these silicon nitride-based coatings.

This study attempts to address the concern of dissolution rate as well as coating adhesion of silicon nitride coatings. We hypothesized that alloying with chromium and niobium would affect the adhesion, dissolution rate, and the resulting ion release and cell response to the coatings. A combinatorial approach was used to deposit sputtered coatings with compositional gradients both with and without a CrN interlayer. 

Compositional gradients were achieved for all the investigated elements: Si (38.6-46.9 at.%), Nb (2.2-4.6 at.%) and Cr (1.9-6.0 at.%). However, while the presence of an interlayer reduced the delamination during adhesion testing, the differences in composition in the top coating did not affect the adhesion. Nor did the top coating’s composition affect the surface roughness or the coatings’ inherent mechanical properties (elastic modulus and hardness). All coating compositions were associated with a low Co release from the underlying metal and points with a higher Cr content (4.3-6.0 at.%) gave an overall lower release of Si, Cr and Nb ions, possibly due to the formation of a stable oxide, which reduced the dissolution rate of the coating. Optimum chromium contents were furthermore found to give an enhanced in vitro fibroblast cell response.

In conclusion, the results indicate a possibility to tailor the ion release rate, which lends promise to further investigations such as tribocorrosive tests towards a future biomedical application.